1. Field of the Invention
This invention relates to a process for preparing a nonconductive substrate for electroplating. Further, this invention relates to an improved process for preparing the through hole walls of a printed wiring board (PWB) for electroplating. Still further, this invention relates to an aqueous neutralizer/conditioner solution.
2. Brief Description of Prior Art
For the past thirty years the printed wiring board industry has relied on the electroless copper deposition process to prepare through hole walls in printed wiring boards for electroplating. These plated through hole walls are necessary to achieve connections between two metal circuit patterns on each side of a printed wiring board or, in addition to this, between the inner layer circuit patterns of a multilayer board.
The electroless deposition of copper onto the through hole walls typically consists of precleaning a PWB and then processing according to the following sequence of steps:
Step 1. Preactivator PA1 Step 2. Pd/Sn Activator PA1 Step 3. Rinse PA1 Step 4. Accelerator PA1 Step 5. Rinse PA1 Step 6. Electroless Copper Deposition PA1 Step 7. Electroplating PA1 (a) contacting said substrate surface with an alkaline permanganate solution for an effective time and at an effective concentration and at an elevated temperature to prepare the substrate surface for a metal layer to be later electroplated thereto; PA1 (b) then contacting said substrate surface with an aqueous neutralizer/conditioner solution, said solution comprising water, at least one neutral or acidic reducing agent, and at least one polyelectrolyte polymer conditioner; PA1 (c) then contacting said substrate surface with a liquid dispersion of carbon black comprised of: PA1 (d) separating substantially all of the liquid dispersing medium from said applied dispersion, thereby depositing said carbon black particles in a substantially continuous layer on said nonconducting substrate surface; and PA1 (e) electroplating a substantially continuous metal layer over the deposited carbon black layer on said nonconducting substrate surface.
These processed boards may also be photoimaged before the electroplating process. Typically, the deposited copper layer on each through hole wall is about 1.+-.0.2 mil thick.
Conventional electroless processes have several commercial disadvantages. They require a relatively long process time. The multiple treatment baths have complex chemistry which may require constant monitoring and individual ingredients which may require separate replenishment. The palladium/tin activator also may require expensive waste treatment. Furthermore, these electroless process baths may be very sensitive to contamination. Finally, the multiplicity of rinse baths may require large amounts of water.
Prior to the electroless method of plating through holes, graphite was employed to prepare the walls of the through holes for plating. For example, U.S. Pat. No. 3,099,608, which issued to Radovsky et al. on July 30, 1963, teaches a process for preparing the through hole walls of printed circuit boards for electroplating by initially depositing in said through holes a thin electrically nonconductive film of palladium metal in at least a semi-colloidal form. The patent discloses that graphite had been used previously as a conductive layer for electroplating thereon. See column 1, lines 63-70 and column 4, line 72 to column 5, line 11. These patentees noted several deficiencies with that graphite process including lack of control of the graphite application, poor deposit of the resultant electroplated metal, nonuniform through hole diameters, and low electrical resistance of the graphite.
U.S. Pat. No. 3,163,588, which issued to Shortt et al. on Dec. 29, 1964, also mentions that graphite or its equivalents may be employed to render through hole walls of electric circuit boards conductive for later electroplating metals thereon. See column 3, line 45 to column 4, line 2.
U.S. Pat. No. 4,581,301, which issued to Michaelson on Apr. 8, 1986, teaches the application of a seed layer of conductive particles, such as "carbon", on the walls of through holes before electrolytically plating copper over the seed layer. This reference does not explicitly teach the use of a continuous layer of carbon black dispersion in the seed layer, and does not recognize the advantage of using very small particles of carbon black such as presently claimed. See column 7, lines 63-66 which refer to particles passing through a 400 mesh screen. A 400 mesh screen is equivalent to about 35 microns.
Separately, graphite has been employed in numerous processes for preparing a nonconducting material for a metal coating or plating. For example, U.S. Pat. No. 409,096, which issued to Alois Blank on Aug. 13, 1889, teaches a process for applying copper to asbestos roofing material which comprises first applying powdered plumbago (graphite) in a volatile liquid such as varnish to the surface of the asbestos, then evaporating the volatile liquid to coat the asbestos fibers with fine particles of plumbago. The plumbago coated asbestos sheets are then immersed in a copper electroplating solution and electric current is applied to the coated asbestos sheet to form a thin film of copper thereon. The copper coated sheet is then immersed in a bath of molten metal such as tin, lead, or zinc, and is then removed from the molten bath to effect solidification of the molten metal. The resulting metal coated asbestos sheet is described as being relatively flexible, a nonconductor of heat and substantially fireproof.
U.S. Pat. No. 1,037,469, which issued to Goldberg on Sept. 3, 1912, and U.S. Pat. No. 1,352,331, which issued to Unno on Sept. 7, 1920, disclose processes for electroplating nonconducting materials by first coating the nonconducting material with wax, then coating the wax with a slurry of finely divided particles of graphite or other metal, followed by electroplating of the dust-coated surface with copper or other metal. Neither of these processes are particularly suitable for use in coating the hole walls of circuit boards because the holes are normally extremely narrow in diameter and immersing in wax would tend to plug the hole and prevent coating the hole walls with an electroplating material.
U.S. Pat. No. 2,243,429, which issued to Laux on May 27, 1941, discloses a process for electroplating a nonconductive surface by "graphiting" a thin layer onto the nonconducting surface followed by applying a copper layer electrolytically and "finally a further electrolytic deposit of another metal" is placed thereon.
Separately, carbon black formulations have been employed as conductive coatings for nonconductive materials. For example, U.S. Pat. No. 4,035,265, which issued to Saunders on July 12, 1977, discloses conductive paint compositions containing both graphite and carbon black along with air-hardenable binder. These paints are suitable for application to the walls of a building for use as a heating element.
U.S. Pat. No. 4,090,984, which issued to Lin et al. on May 23, 1978, teaches a semi-conductive coating for glass fibers comprising (a) a polyacrylate emulsion; (b) electrically conductive carbon black dispersion and (c) a thixotropic gelling agent. The conductive carbon black dispersions employed are those comprising electrically conductive carbon black dispersed in from about 3 to about 4% by weight of a suitable dispersing agent.
U.S. Pat. No. 4,239,794, which issued to Allard on Dec. 16, 1980, teaches dispersing a conductive carbon black in a latex binder with a selected dispersing agent, then impregnating this carbon black dispersion into a nonwoven fibrous web followed by drying any residual water, leaving a thin coating of carbon black dispersed on the surfaces of said fibers.
U.S. Pat. Nos. 4,619,741; 4,684,560 and 4,724,005, which issued to Karl L. Minten and Galina Pismennaya, on Oct. 28, 1986; Aug. 4, 1987; and Feb. 9, 1988, respectively, teach a process of electroplating the through holes of a PWB which is a significant improvement over the known electroless techniques. By this process, a liquid dispersion of carbon black particles is first applied to the nonconductive portions of the through holes; then the liquid dispersion medium is separated (i.e., evaporated) from the carbon black particles, thereby depositing a substantially continuous layer of carbon black particles on the nonconductive surfaces of the through holes; and next a substantially continuous metal layer is electroplated over the deposited carbon black layer. This process of Minten and Pismennaya has several advantages over the known electroless techniques including the elimination of the preactivator, the Pd/Sn activator and the accelerator; less possibility of pollution problems; better bath stability; and fewer possible side reactions. This disclosure of the above-mentioned U.S. Patents of Minten and Pismennaya is incorporated herein by reference in their entirety.
Improvements and modifications of this Minten and Pismennaya process are shown in U.S. Pat. No. 4,622,107 (Piano); U.S. Pat. No. 4,622,108 (Polakovic and Piano); U.S. Pat. No. 4,631,117 (Minten, Battisti, and Pismennaya); and U.S. Pat. No. 4,718,993 (Cupta and Piano); U.S. Pat. No. 4,874,477 (Pendleton) and U.S. Pat. No. 4,897,164 (Piano and Galvez). The first of these patents teaches the use of a gas-forming compound (e.g. sodium carbonate) to remove loose or easily removable carbon black particles in the through holes. The second of these patents teaches the contacting of an alkaline hydroxide preconditioning solution to the through hole walls before application of the carbon black dispersion so that the carbon black dispersion will have better adhesion to the walls. The third listed patent teaches the use of this carbon black dispersion as a preactivator for electroless plating of the through holes. The fourth teaches the use of an alkaline silicate solution before the carbon black dispersion. The fifth patent teaches the use of an aqueous polyelectrolyte homopolymer conditioner solution before the carbon black dispersion bath. The sixth patent teaches the use of an alkaline borate solution to remove excess carbon black material on the rims and inner metal walls of the PWB through hole walls which might cause an undesirable, uneven plated surface to result. These six U.S. Patents are incorporated herein by reference in their entireties.
One problem present with multilayer PWB through holes is that the drilling of the holes causes resin smear on the exposed conductive metal (e.g., copper) inner layers on the holes. The resin smear may act as an insulator between the later plated-on metal in the through hole and these inner metal layers. Thus, this smear may result in poor electrical connections. The smear should be removed (i.e., "desmeared") before the plating-on operation.
Various alkaline permanganate treatments have been used as standard methods for desmearing surfaces of printed wiring boards including the through holes of printed wiring boards. Such permanganate treatments have been employed for reliably removing smear/drilling debris and texturizing or micro-roughening exposed PWB epoxy surfaces. This latter effect significantly improves copper-to-epoxy resin adhesion.
Generally, permanganate treatment involves three different treatment solutions used sequentially. They are (1) a solvent swell solution, (2) a permanganate desmear solution, and (3) a neutralization solution. Typically, a printed wiring board is dipped or otherwise exposed to each solution with deionized water rinse baths employed between each of these three treatment solutions.
Numerous U.S. and foreign patents and published foreign patent applications have issued which teach different permanganate desmearing and neutralization compositions and/or desmearing or neutralization operations. U.S. Pat. No. 3,962,496 (Leech) teaches of a hydrazine neutralizer solution containing a sequestering agent (e.g. ethylenediamine tetraacetic acid, sodium tartrate, and triethanolamine) and a pH adjustor (e.g. sodium hydroxide, potassium hydroxide, and sodium carbonate).
U.S. Pat. No. 4,042,729 (Polichette et al.) and U.S. Pat. No. 4,073,740 (Polichette et al.) teach a composition for treating a resinous surface to later receive a deposit of electrolessly-formed metal, said composition comprising water, permanganate ion and manganate ion, wherein the molar ratio of manganate ion to permanganate ion is up to 1.2 to 1 and said composition having a pH in the range of 11 to 13.
U.S. Pat. No. 4,054,693 (Leech et al.) teaches a process of treating a resinous surface by first contacting that surface with the same permanganate ion/manganate ion solution as used in the preceding two patents, then neutralizing the treated resin surface with an aqueous solution comprising hydrazine and then following that neutralization with metallizing that resinous surface with an electroless metal deposition bath.
U.S. Pat. No. 4,233,344 (Braach) teaches treating a composite substrate with a copper-type colloidial system to cause activation of the nonconductive portions thereof for electroless metal deposition, and thereafter treating the activated substrate with an adhesion promoter (i.e., hydrazine hydrate, ammonium persulfate, or alkali hydroxide) prior to electroless metal deposition.
U.S. Pat. No. 4,425,380 (Nuzzi et al.) teaches a process for preparing a resinous substrate for subsequent metallization, said process comprising first contacting the substrate with an alkaline permanganate treating solution, then contacting said substrate with a water-soluble compound (e.g., tin chloride, sodium bisulfite, hydrochloric acid, or hydroxylamine hydrochloride) to reduce any manganese residues deposited on said substrate to a low oxidation state, and finally contacting said substrate with an alkaline hydroxide solution to remove essentially all of said manganese residues.
U.S. Pat. No. 4,430,154 (Stahl et al.) teaches a process for making printed circuit boards involving the steps of removing an adhesive coating by treating the board with an aqueous solution containing potassium permanganate and sodium hydroxide, and thereafter treated with an aqueous solution of hydrochloric acid or hydrazine hydrate.
U.S. Pat. No. 4,515,829 (Deckert et al.) teaches an overall process for manufacturing a printed circuit board having a plurality of metal plated holes interconnecting at least two circuits, including the steps of drilling holes in an epoxy board, forming the circuits, contacting the hole walls with an aqueous alkaline oxygenated epoxy solvent at a pH greater than 10, then contacting the holes with an aqueous alkaline permanganate solution at an elevated temperature and a pH in excess of 13, and also contacting the hole walls with a reducing agent solution.
U.S. Pat. No. 4,592,852 (Courduvelis et al.) teaches an alkaline composition to improve the adhesion of plastics to electroless metal deposits, said composition containing a source of permanganate ions and a secondary oxidant selected from the group consisting of chlorine, bromine, ozone, hypochlorite salts, metaperiodate salts and trichloro-s-triazinetrione salts.
U.S. Pat. No. 4,592,929 (Tubergen et al.) teaches a process for the metallization of a plastic which includes the steps of first treating the plastic with a liquid permanganate oxidant solution, then contacting the plastic with a solution containing a reducing agent, a pH adjustor to render the reducing agent active, and a surface active agent in sufficient concentration to reduce the surface tension of the solution to 50 dynes per centimeter or less.
U.S. Pat. No. 4,601,784 (Krulik) teaches an aqueous alkaline sodium permanganate solution comprising water, an alkali metal hydroxide, sodium permanganate, and 0.1 to about 3.0 moles of K.sup.+, Cs.sup.+, Rb.sup.+ ions, or mixtures thereof, per mole of permanganate ion.
U.S. Pat. No. 4,629,636 (Courduvelis et al.) teaches a process to improve the adhesion of a plastic to an electroless metal deposit wherein said plastic is contacted with an alkaline permanganate solution which contains permanganate ions, manganate ions, and a secondary oxidant; the secondary oxidant being added at controlled intervals to keep the ratio of permanganate ion concentration to the sum of permanganate and manganate ion concentrations above about 0.5.
U.S. Pat. No. 4,698,124 (Krulik) teaches a method for regenerating spent permanganate ions in a permanganante-containing etchant composition comprising periodically adding an oxidizer selected from the group consisting of an inorganic peroxy disulfate, mixtures of an inorganic peroxy disulfate, and an inorganic hypochlorite, and mixtures of an organic peroxy disulfate, and an inorganic chlorate in an amount to oxidize essentially all of the nonpermanganate manganese species in the composition to permanganate.
Japanese Patent No. 81-003373 (which issued on Jan. 24, 1981) and Japanese Patent No. 81-015736 (which issued on Apr. 11, 1981) teach the use of alkaline solutions of potassium permanganate and sodium or potassium hypochlorite in the treatment of ABS resins prior to electroless metal plating. The alkaline solutions include those having a pH in the range of 11.0 to 12.35 and 12.0 to 13.5, respectively.
Japanese Kokai No. 79-055933 and Japanese Kokai 79-117,328, the latter published on Sept. 12, 1979, teach an electroless plating on plastics process involving etching the plastics with aqueous solution containing potassium permanganate and persulfate prior to electroless metal plating. All of the above U.S. and foreign patents and patent publications are incorporated herein by reference in their entireties.
It is a primary object of this invention to provide an improved electroplating process for applying a conductive metal layer to a nonconducting material such as the through hole walls of printed wiring boards over the process disclosed in the above-noted Minten and Pismennaya patents.
It is also an object of the present invention to provide a unified permanganate desmearing operation with the carbon black dispersion preplating operation disclosed in the above-noted Minten and Pismennaya patents.
It is another object of the present invention to provide a unified permanganate desmearing/carbon black dispersion preplating operation whereby the neutralization treatment of the desmearing operation and the conditioning treatment of the preplating operation is combined into one step.
It is still another object of this invention to provide an even more economical and environmentally safe process for applying a conductive metal layer to the surfaces of nonconducting layers of printed wiring boards than presently known combined permanganate/electroless processes.